One-time programmable memory and method for making the same

ABSTRACT

A one time programmable nonvolatile memory formed from metal-insulator-semiconductor cells. The cells are at the crosspoints of conductive gate lines and intersecting doped semiconductor lines formed in a semiconductor substrate.

TECHNICAL FIELD

This invention relates to a nonvolatile programmable semiconductormemory, and more particularly, to the making and operating of a one-timeprogrammable (OTP) anti-fuse memory.

BACKGROUND

Nonvolatile memory retains stored data when power is removed and isdesirable in many different applications. As system-on-chips (SoCs)become more prevalent in consumer electronics and industrialapplications, embedded nonvolatile memories have become more common.Embedded memory is incorporated onto the same underlying semiconductordie and non-memory circuitry.

The embedded memory is used for various purposes, among which are chipIDs, analog trimming, yield enhancement, and code storage. It would beadvantageous if the embedded memories did not require added masks andprocess modifications to a standard CMOS flow. “Flash” memory that usesmultiple polysilicon layers is not compatible with standard CMOS flow.As a result, gate dielectric based anti-fuse memory increasingly hasbecome the choice of SoC chip designers because it is standard CMOSprocess based, reliable, and secure.

Gate dielectric anti-fused based memory can be broadly categorized intotwo groups, depending upon its operating principle. The first type is across-point memory consisting of a single capacitor at each gridpoint.The second type has more than two access lines for each cell in thememory array. A typical example is a storage capacitor or transistorcoupled in series with a selection device such as a transistor or diode.examples of the first type can be found in U.S. Pat. Nos. 6,898,116,6,992,925, 7,638,855, and 7,110,278. An example of the second type isU.S. Pat. No. 6,667,902 (and the references cited therein).

Cross-point memory arrays are advantageous due to its compact layout andsimple decoding. As a result, embedded OTP memories of this type can beabout eight times smaller than those of the second type. However, priorart cross-point OTP memories have drawbacks, such as significant processcomplexity, array leakage current, and reliability.

Furthermore, for embedded applications, it is very important to complywith logic layout design rules while introducing no extra process stepsor only non-critical ones. As shown in prior art FIG. 1 (FIG. 2 of U.S.Pat. No. 7,638,855 to Lung), disclosed is a cross-point antifuse memorythat requires significant changes in standard CMOS process flow andneeds additional critical implant masks because the N+ bit lines andP-isolations are not self-aligned. In addition, the gate dielectricbefore programming and the P+/N+ diode formed after programming can havequestionable quality.

U.S. Pat. Nos. 6,898,116 and 6,992,925, as illustrated in prior art FIG.2 (FIG. 28 from the '925 patent), attempted to solve these problemsusing standard MOSFETs by adding buried N+ or P+ bodies. In the '925patent, there are source and drain regions that extend under thesidewall spacers, thereby connecting to the channel region under thegates. Due to the presence of source and drain regions, however, thereare two potential disadvantages with this cell. First, program disturbfrom inhibit voltages applied to the body can occur for un-selectedcells where the gate is biased at zero voltage and body at Vpp. Due toimpact ionization and other high voltage mechanisms, the floatingsource/drain can be charged up to a voltage well above ground. As aresult, the MOSFET device can be fully inverted and a large percentageof the inhibit voltage drops across the gate dielectric. Secondly, thegate dielectric may breakdown at the overlap region between the gate andLDD. When this happens at two neighboring cells, there will be a pathfor leakage current during both programming and read operations.

U.S. Pat. No. 7,110,278 to Keshavarzi discloses a cross-point memorysimilar to that of Peng except that the source and drain of each MOSFETis disconnected from its neighbors, as shown in prior art FIG. 3 (FIG. 2of the '278 patent). The cell is bigger as a result of thenon-continuous active regions. Furthermore, program disturb from thebody can remain a problem because source and drain doped regions arestill present for each MOSFET transistor.

Consequently, there is a need for a cross-point anti-fuse OTP memorythat offers not only a compact size but also logic CMOS compatibility,low leakage current, and improved program reliability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 are prior art nonvolatile memory cells.

FIG. 4 shows schematically an unprogrammed memory array.

FIG. 5 shows a cross section of a memory cell of the memory array ofFIG. 4.

FIG. 6 shows a top plan view of a memory array.

FIG. 7 is a flow diagram of the process steps for manufacturing thememory array.

FIG. 8 shows a cross-section view of the memory array of FIG. 6 takenalong lines A-A′ and B-B′.

FIGS. 9-10 shows a second embodiment of a memory array.

FIGS. 11-12 shows a third embodiment of a memory array.

FIGS. 13-14 shows a fourth embodiment of a memory array.

FIGS. 15-16 shows a fifth embodiment of a memory array.

FIG. 17 shows a memory array during programming and reading.

DETAILED DESCRIPTION

Various embodiments of the present invention are now illustrated infollowing figures using terms commonly employed by those skilled in theart. It will be understood that they are not intended to limit theinvention to these embodiments. The invention can be practiced withoutone or more of the specific details, or with other methods, components,materials. In other instances, well-known structures, materials, processsteps, or operations are not shown or described in detail in order notto obscure aspect of the invention.

FIG. 4 illustrates a 3×3 cross-point memory array wherein each memorycell MC is shown before programming. Wordlines (WL) are positioned inthe horizontal direction and bitlines (BL) in the vertical direction.Note that the orientation and terminology used to describe the lines maybe switched or different terminology used altogether. A memory cellconsisting of a Metal-Insulator-Semiconductor (MIS) capacitor is locatedat each cross point. Note that while the term “metal” is used in MIS,the metal in many embodiments is actually doped polysilicon—in thecontext of the present disclosure, the term metal is meant to includeany and all conductive structures. The WL and BL are so named forconvenience only and they are, for example, referred to as rows (R) andcolumns (C) interchangeably in this specification. Further, the arraymay be of arbitrary size m by n, where m or n ranges, in one embodiment,from 1 to 1024, but may be larger.

FIG. 5 shows a cross-sectional view of two different types of the MISmemory cell. FIG. 5( a) is for an n-type MIS cell where the body isp-type and the gate is of n-type conductivity. FIG. 5( b) is for ap-type MIS cell where the doping is opposite to that of FIG. 5( a). Itshould be noted that the cells have a gate stack which are the same asthat of a standard MOSFET. However, there are no source/drain implants(charge reservoirs) physically connected to the channel. The gatedielectric can be of any commonly used materials in the industry such asoxide, nitride, oxynitride, and other high dielectric constantmaterials. For convenience, p-type doped poly gate conductors will beused to describe various embodiments. In practice, n-type doped poly ormetal gate can be implemented as well.

In accordance with a disclosed embodiment, FIG. 6 shows a partial layoutdiagram of a 3×3 array. For simplicity, only a few relevant layers aredrawn. Active stripes are formed in vertical stripes in the substrate.The active stripes are formed by a doping implant. For a p-type dopedpoly gate conductor, the implant would be an n-type implant into ap-type substrate. A typical doping concentration for the n-type bitlinesBL could range from 1×10¹⁸ to 1×10¹⁹. In one embodiment, the width ofthe stripes range from 1× to 2× of the minimum feature size (F) that canbe patterned at a given technology node. Formed between the activestripes are isolation regions, which may be, as an example, modifiedLOCOS or shallow trench isolation (STI). The width of the isolationregions between active stripes may also range between 1× to 2× of theminimum feature size F.

Still referring to FIG. 6, arranged in horizontal stripes are gateconductor wordlines WL. Like the bitlines BL, the WL width and spacingtherebetween ranges from 1× to 2× of the minimum feature of a givenprocess technology. Thus, for high density applications, the cell sizecan be as low as 4F², assuming that the width of the BL, WL, andisolation spacings are all at the minimum feature size.

As noted, the embodiments disclosed herein follow standard CMOS processflow except for the addition of a bitline BL implant mask that is usedto form the Wines (active stripes) in the substrate. FIG. 7 shows aprocess flow that may be used to manufacture the disclosed embodiments.First at box 701, a standard n-well implant is performed generally inthose areas outside of the memory array. Those of skill in the artrecognize that n-wells are conventionally formed in a p-type substratein a CMOS process.

While the standard n-wells are being implanted, the memory array regionsare masked off in addition to the n-MOSFET devices. Thus, the drawnlayer CBI serves two purposes: (1) to generate the n-well mask such thatthe memory array is covered while n-wells are being implanted in otherareas of the substrate, and (2) to generate a bitline mask to form theactive stripes. In some embodiments, this process may be performed bythe combination of the n-well and bitline implants.

Next at box 703, the active stripes are implanted by the n-type dopant.This could be done with phosphorus and/or arsenic with a dose rangingfrom 1×10¹⁴ to 1×10¹⁵ and an energy ranging from 20 KeV to 80 KeV. Aswill be seen below, the active stripes in one embodiment have a superretrograde profile such that there is a deeper n+ band near the bottomand a shallower n− region near the surface. Depending on the particularprocess technology, well known multiple dose and energy implants can beused just like those used to form the standard n-well. The active stripeimplant (also referred to as a cell bit implant (CBI)) may be doneeither before or after the regular n-well implant, without extra thermalannealing. In this embodiment, the implant is n-type dopant, similar tothe n-well implant, but with a lower energy.

It is desirable to have the CBI: (1) have its n-p substrate junctionshallower than the isolation STI (see FIGS. 8), and (2) have a superretrograde profile so that the BL resistance is low. For example, areasonable value is about 500 Ohm to 3 KOhm between two BL strapcontacts. Those skilled in the art know that the standard processmodules such as the shallow trench isolation (STI), p-well implant, wellannealing, and other processes are skipped for clarity and referred asthe standard CMOS flow.

FIG. 8( a) is a cross-sectional view of FIG. 6 taken along A-A′. Due tothe use of lower energy implants, BLs consists of heavily doped n+regions near BUp-Sub junction and lightly doped n− regions near the gatedielectric interface. The bitlines BL are separated by STI isolations(though other isolation structures may be used) so that there is noleakage between BLs.

FIG. 8( b) is a cross-sectional view of FIG. 6 taken along the lineB-B′. Note that the cross section is different from standard PMOSFETsdue to the removal of LDD/HALO implants, as shown in box 707 of FIG. 7.P+ doped regions formed in the substrate can be as a result of thestandard p+ source/drain implant self-aligned to the sidewall spacers.Note that they are electrically floating and not physically connected tothe channel regions. Unlike the prior art, the p+ regions do not extendto the gate, and thus are not in electrical contact with a channelregion under the gate. The sidewall spacers on the gates separate the p+regions from the channel.

Indeed, as noted above, the p+ floating regions are not part of theactive cell devices and therefore are optional (and can be masked out).However, to avoid additional masking steps, they can be left in (sincethey are floating and electrically isolated) and are formed fromself-aligned source/drain implant when standard CMOS poly gate designrules are used.

One way to eliminate the optional p+ floating regions is illustrated inFIGS. 9 and 10, which show another embodiment of the memory array. Herethe gate spacing is so designed such that when standard sidewall spacerdeposition is performed, the sidewall spacers conformally fills thespace between adjacent wordlines WL. As a result, as seen in FIG. 10(b), the space between adjacent gates are substantially filled afterspacer etch. This prevents the p+ source/drain implants from reachingthe semiconductor substrate. Cross-sectional views along both A-A′ andB-B′ of FIG. 9 are shown in FIG. 10. As seen in FIG. 10( b), there areno p+ regions in the substrate. The benefit of this cell is a morecompact array with a potential cell size of 4F².

Yet another embodiment is illustrated in FIGS. 11 and 12. The memorycell can be made from standard dual-oxide CMOS processes. In thisembodiment, the gate oxide underneath the gate has a thicker region anda thinner region. The gate dielectric formed under a thicker gatedielectric mask is used to grow a thicker gate dielectric 1101, whichcan be the same as that of standard I/O oxide. The objective is tofurther restrict the breakdown locations away from the gate edges sothat cell to cell sneak leakages can be significantly reduced. Anexample of the use of a thicker gate oxide is shown in commonly assignedU.S. Pat. No. 6,940,751, which is herein incorporated by reference.Cross-sectional views are shown in FIG. 12.

It can be appreciated that various combinations of the multiple conceptsdescribed herein may be combined into yet other embodiments. Forexample, the thicker gate oxide technique may be combined with theblocked source drain implant of FIGS. 9 and 10.

Still, in yet another embodiment, the floating doped semiconductorregions can be n+-type. As shown in FIGS. 13 and 14, a channel stopimplant layer 1301 is used to block the p+ source/drain implant and toopen an n+ channel stop implant 1401. Cross-sectional views are given inFIG. 14. Although this structure provides even better cell to cellleakage current protection after they are programmed, it does requireextra process steps and the addition of critical implant masks, with theassociated alignment tolerance issues.

For OTP memories of smaller capacity, the memory array itself is arelatively small percentage of the total die area. In these embeddedapplications, it is advantageous to develop antifuse memories withoutintroducing added mask and process steps in addition to standard CMOSprocesses. As such, yet another embodiment eliminates the additional CBImask described above. FIG. 15 shows a layout view of this embodiment.

In this embodiment, the bit line implant 1501 is the standard n-wellimplant mask. Instead of covering the whole memory array area, then-well implant mask covers each active stripe 1503 individually. N-wellspacing is designed to prevent BL to BL leakage during programming. Thecell size of this embodiment is larger than the others because theregular n-well is deeper than that of STI. Cross-sectional views aregiven in FIG. 16.

Note that the above embodiments are for p-type MIS cells and can beeasily switched to n-type MIS cells. Programming and read operations arethe same for all p-type implementations. A simple polarity changeapplies to all n-type MIS cell embodiments.

With FIG. 17 as a reference for a p-type cell implementation, Table 1below provides example bias conditions for both programming and readoperations. The cell marked by ‘Sel A’ is assumed to be the selectedcell for both program and read. Here the program Vpp and read Vread arefor example only and their actual levels depend on the specific processtechnology used. For gate dielectrics with thickness of 6 nm to 32 nm,Vpp and Vread are preferred to be in the range of 3V·9V and 0.7V˜3.3V,respectively. For the selected cell ‘A’, the capacitor is underaccumulation and the full Vpp is applied across its gate dielectric. Itsgate dielectric breaks down and the cell is programmed.

For an un-selected cell at (WLi, BLn), the MIS capacitor is under deepdepletion and the cell will not be disturbed. For the un-selected cellat (WLi, BLI), the programmed cell behaves as a reverse biased diode andits leakage current is extremely small. There is no effective voltagedeveloped across MIS cells at (WLj, BLI) and (WLj, BLn). During readoperations, bias conditions are similar to those of programming exceptthe change from Vpp to Vread.

TABLE 1 Operation WLi WLj WLk BLl BLm BLn Program 0 V or Vpp 0 V or Vppor 0 V Vpp or Floating Floating Floating Floating Read 0 V or Vread 0 Vor Vread or 0 V Vread or Floating Floating Floating Floating

Features and aspects of various embodiments may be integrated into otherembodiments, and embodiments illustrated in this document may beimplemented without all of the features or aspects illustrated ordescribed. One skilled in the art will appreciate that although specificexamples and embodiments of the system and methods have been describedfor purposes of illustration, various modifications can be made withoutdeviating from the spirit and scope of the present invention. Moreover,features of one embodiment may be incorporated into other embodiments,even where those features are not described together in a singleembodiment within the present document. Accordingly, the invention isdescribed by the appended claims.

I/We claim:
 1. An antifuse-based one-time programmable non-volatilememory cell comprising: a buried bitline formed in a substrate, theburied bitline of a first conductivity type; a dielectric layer formedover at least a portion of the buried bitline; and a conductive gateformed over the dielectric layer, the conductive gate defining a channelregion under the conductive gate and dielectric layer; wherein thechannel region does not have electrical interaction other than to saidburied bitline or conductive gate.
 2. The memory cell of claim 1 whereinformed on the sidewalls of the conductive gate are sidewall spacers. 3.The memory cell of claim 2 wherein floating regions of a secondconductivity type are formed in the substrate spaced away from thechannel region by the sidewall spacers.
 4. The memory cell of claim 2wherein regions of higher dopant concentration of the first conductivitytype are formed in the bitline spaced away from the channel region bythe sidewall spacers.
 5. The memory cell of claim 1 wherein the buriedbitline has a graded dopant concentration with a lower dopantconcentration near the dielectric layer and a higher dopantconcentration deeper in the substrate.
 6. The memory cell of claim 1wherein said dielectric layer is thicker proximal to at least a portionof the edge of the channel region than to the center of the channelregion.
 7. The memory cell of claim 1 wherein the buried bitline isformed from standard n-well implants.
 8. A memory array comprised of aplurality of antifuse-based one-time programmable non-volatile memorycells, the memory array comprising: a plurality of buried bitlinesformed in a substrate, the buried bitline of a first conductivity type;a dielectric layer formed over at least a portion of the buriedbitlines; and a plurality of conductive gate wordlines formed over thedielectric layer, the conductive gate wordlines intersecting with theplurality of buried bitlines, the memory cells located at theintersection of said conductive gate wordlines and buried bitlines,further wherein a channel region is defined under the intersection ofsaid conductive gate wordlines and buried bitlines and dielectric layer;wherein the channel region does not have electrical interaction otherthan to said buried bitline or conductive gate.
 9. The memory array ofclaim 8 wherein formed on the sidewalls of the conductive gate of thememory cells are sidewall spacers.
 10. The memory array of claim 9wherein floating regions of a second conductivity type are formed in thesubstrate spaced away from the channel region by the sidewall spacers.11. The memory cell of claim 7 wherein regions of higher dopantconcentration of the first conductivity type are formed in the bitlinespaced away from the channel region by the sidewall spacers.
 12. Thememory array of claim 8 wherein the buried bitlines have a graded dopantconcentration with a lower dopant concentration near the dielectriclayer and a higher dopant concentration deeper in the substrate.
 13. Thememory array of claim 8 wherein said dielectric layer is thickerproximal to at least a portion of the edge of the channel region than tothe center of the channel region.
 14. The memory array of claim 9wherein the sidewall spacers of adjacent conductive gate wordlinescompletely span the space between the adjacent gate wordlines.
 15. Thememory array of claim 8 wherein shallow trench isolations are formedbetween the burled bitlines.
 16. The memory array of claim 8 whereinburied bitlines are formed from standard n-well implants.